crackling

I recorded a full drum set and bass (8 tracks) the other day in my new rig, no problem. I might've messed with some settings accidentally, I don't know. But when I went to do an overdub of one guitar track on a few songs, they all came out with intermittent crackling. Nothing was peaking. Seems like a resolution setting is wrong or something. Clues?
 
I'm running on Win 7.